Detection of liquid and gaseous water is important in many industries, including pharmaceuticals, microelectronics fabrication, food processing, and chemical manufacturing. Electronic sensors based on chemiresistors, microcantilevers, and surface acoustic wave (SAW) devices have been demonstrated and are capable of detecting water vapor at sub-ppb levels in real time. Such sensors generally require power to operate and potentially cumbersome data collection and analysis (either in hardware or software) to produce a result. In some applications, these are disadvantages, particularly in cases with limited power accessibility or limited space availability. Another disadvantage is that not all water-sensitive coatings used to enhance sensitivity and/or selectivity are stable in both humid environments and liquid-phase water.
Although electronic sensors can provide real-time information and high sensitivity, often the only information required is a positive or negative indication that water is present. In this case, a simple visual indicator is highly desirable. For example, some versions of Drierite™ (anhydrous calcium sulfate or gypsum) are mixed with a small amount of cobalt(II)chloride, which changes color from blue to pink when exposed to moisture. 3M markets “Water Contact Indicator Tape,” which changes from white to red when in contact with liquid water. Disadvantages of these materials are their relatively limited capacity (10-14 wt % for Drierite™) and inability to detect both liquid and vapor-phase water.